Add an optional configuration option to set 'Precedence: bulk' headers on
transactional mail

Summary: See T571. SES refuses to deliver mail with this header and there are
various reports of other issues on the internet so I'm defaulting it to off.

Test Plan: Set config to true, tried to send mail, SES rejected it because of
"Precedence: bulk" header.

Reviewers: bmaurer, ola, jungejason, nh, aran

Reviewed By: aran

CC: aran, epriestley, bmaurer

Differential Revision: 1032